FIG. 1 illustrates a prior art switching power supply. The system of FIG. 1 includes a modulator 20 that generates switching control signals SC1 and SC2 to drive switch circuits 10 and 12, thereby controlling the amount of power delivered to the load 22 through inductors 14 and 16. An output filter 21 includes a series resistor/capacitor combination to filter the output from the inductors. A voltage mode error amplifier circuit 17 generates an error signal VERR in response to the output voltage VOUT so the modulator can modulate the switch signals to maintain a constant output voltage regardless of the amount of current consumed by the load. The sensed output voltage is combined with an input signal Vref to generate the error signal that is applied to the modulator for closed-loop control of the output. The modulator 20 shown in FIG. 1 is assumed to provide pulse-width modulation (PWM), but other modulation schemes such as pulse frequency modulation (PFM), hysteretic control (ripple regulation), etc., may be used.
The system of FIG. 1 also includes a current sensing circuit 18 to generate a signal VCS that provides a measure of the total combined output current delivered to the load. The current sense signal may be used in numerous ways. For example, it may be used to provide over-current shutdown, it may be used to implement current-mode regulation, or it may be combined with voltage feedback to establish a droop impedance for adaptive voltage positioning (AVP) control schemes.
The system of FIG. 1 is known as a multi-phase switching power supply because the power components including the switches and inductors are repeated to produce multiple output currents that are summed together to provide the total output current. This increases the amount of current available from the power supply.
Although the circuit of FIG. 1 provides good regulation and transient response in many applications, switching power supplies for microprocessors are subject to ever more demanding performance requirements. For example, under certain operating conditions, the supply current demanded by a high performance processor may drop from full load down to 30 percent and then immediately go back up to full load at frequencies of hundreds of kilohertz (kHz). The power supply must be able to supply these rapidly changing currents while still maintaining the supply voltage within a very narrow range.
An example of a problem caused by high frequency load transients is saturation of the error amplifier. That is, when the output load current changes rapidly from a low to high or high to low level, the output signal from the error amplifier changes rapidly to force the PWM output to compensate for the given load change. Since the error amplifier is non-ideal, it has some limited minimum and maximum voltage range that can be reached if there is a large error signal generated at the output, as would be the case for very large output load changes. This may drive the error amplifier into saturation with the output at one of the voltage rail limits. Once in saturation, it takes additional time for the error amplifier to swing its output voltage back from the rail once the output voltage of the power supply has come back into regulation.
Some efforts have been made to clamp the output of the error amplifier with a diode. Although this may prevent saturation, it may also cause overshoot in response to an abrupt load change in the opposite direction, thus causing the power supply output to exceed the specified voltage range, and possibly damaging the processor which is very sensitive to supply voltage variations.
Compounding these problems is the asymmetric slew rate of the output inductors as they are typically configured in switching power supplies. PWM based DC power supplies convert an input voltage to an output voltage via a switched inductor and output filter. The input to output ratio determines the system duty cycle. However, the large signal slew rate is typically asymmetric, so large load changes may drive the PWM stage into saturation at its minimum or maximum duty cycle, and this may cause a different response and non-linear operating point depending on the direction of the load change.